Not a direct fit. Joining 3/8" NPT (National Pipe Taper) to 3/8" BSP tapered (R3/8 / BSPT) needs a 3/8 NPT to R3/8 BSP tapered adapter. Sealing method, the real-world trap, and the exact adapter to buy.
Updated 05 September 2026.
Connect 3/8" NPT (National Pipe Taper) to 3/8" BSP tapered (R3/8 / BSPT)
How to join 3/8-18 NPT and R3/8, and what stops it leaking.
Not a direct fit. Joining 3/8" NPT (National Pipe Taper) to 3/8" BSP tapered (R3/8 / BSPT) needs a 3/8 NPT to R3/8 BSP tapered adapter.
The adapter to buy
3/8 NPT and R3/8 BSPT are both tapered 3/8 inch threads, which fools people into forcing them together, but the flank angle differs: 60 degree on NPT, 55 degree on BSPT. They cannot seal against each other, so an NPT-to-BSP-tapered adapter is the correct fix.
Both being tapered at 3/8 inch is the trap. Same taper, wrong flank angle: the joint starts smoothly then leaks. Use the adapter, not extra tape.
Related connections
Questions and answers
Connect 3/8" NPT (National Pipe Taper) to 3/8" BSP tapered (R3/8 / BSPT)?
Not a direct fit. Joining 3/8" NPT (National Pipe Taper) to 3/8" BSP tapered (R3/8 / BSPT) needs a 3/8 NPT to R3/8 BSP tapered adapter.